Problem: Simplify to lowest terms. $\dfrac{21}{35}$
What is the greatest common factor (GCD) of 21 and 35? $21 = 3\cdot7$ $35 = 5\cdot7$ $\mbox{GCD}(21, 35) = 7$ $\dfrac{21}{35} = \dfrac{3 \cdot 7}{ 5\cdot 7}$ $\hphantom{\dfrac{21}{35}} = \dfrac{3}{5} \cdot \dfrac{7}{7}$ $\hphantom{\dfrac{21}{35}} = \dfrac{3}{5} \cdot 1$ $\hphantom{\dfrac{21}{35}} = \dfrac{3}{5}$